Optical transmitter

ABSTRACT

In an optical transmitter comprising a directly modulated laser and a wavelength filter provided on a post-stage of the directly modulated laser, the wavelength filter has a modulated light input port for inputting modulated light output from the directly modulated laser, a filter transmitted light output port for outputting light having a wavelength included in a filter transmission band among the modulated light as filter transmitted light, and a filter cutoff light output port provided separately from the modulated light input port and the filter transmitted light output port and outputting light having a wavelength included in a filter cutoff band among the modulated light as filter cutoff light, and the peak of the filter transmission band is set on a shorter-wave side from the peak of the spectrum of modulated light output from the directly modulated laser.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to JapaneseApplication No. 2005-330372 filed on Nov. 15, 2005 in Japan, thecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an optical transmitter used in anoptical communication system, and for example, to an optical transmitterprovided with a directly modulated laser.

(2) Description of Related Art

An optical transmitter module provided with a directly modulated laser(DML) is simple in comparison with one using an external modulator andhas advantages that the cost is low, the space is saved, and theconsumed power is small.

However, the DML has large wavelength chirping in comparison with oneusing an external modulator and has a disadvantage that the transmissiondistance is reduced as a result.

Here, the wavelength chirping means wavelength fluctuations that occuraccompanying intensity modulation of light. In order to perform longdistance transmission, it is necessary to suppress the wavelengthchirping low.

Recently, there is discussion about an attempt to suppress thewavelength chirping low in order to perform longer distance transmissionusing the DML by optimizing drive methods and performing signal spectrumprocessing by use of a wavelength filter (for example, refer to D.Mahgerefteh et al. “Error-free 250 km transmission in standard fibreusing compact 10 Gbit/s chirp-managed directly modulated lasers (CML) at1550 nm” ELECTRONICS LETTERS 28th Apr. 2005 Vol. 41 No. 9).

In this specification, such a DML is particularly referred to as a lowchirp directly modulated laser or low-chirp DML.

Here, FIG. 18 shows a configuration of a low chirp directly modulatedlaser in the prior art.

As shown in FIG. 18, the low chirp directly modulated laser in the priorart uses a single oscillation mode DFB laser 100 as a directly modulatedsemiconductor laser and the laser light output from the DFB laser 100(DFB laser output light; modulated light) is caused to be inputted to awavelength filter 103 via a coupling lens 101 and an isolator 102.

In this case, part of filter input light inputted to the wavelengthfilter 103 passes through the wavelength filter 103 and is output asfilter transmitted light (module output light) as a result. On the otherhand, part of the filter input light is cut off by the wavelength filter103 and returns to the isolator 102 side as filter cutoff light.

Additionally, the isolator 102 is arranged in order to prevent thecutoff light cut off by the wavelength filter 103 from entering the DFBlaser 100 as returned light.

The drive conditions of the DFB laser 100 in the low chirp directlymodulated laser, the filter conditions of the wavelength filter 103,etc., will be explained in detail below with the principles of the lowchirp directly modulated laser. Here, there is exemplified the casewhere the low chirp directly modulated laser is driven under theconditions that the operation wavelength is 1.55 μm and the modulationrate is 10 Gb/s.

By comparing with a normal DML, the DFB laser 100 in the low chirpdirectly modulated laser is driven under the conditions that the averagelight output is increased by increasing the value of the direct currentbias and the extinction ratio is reduced by reducing the drive currentamplitude value. Normally, the extinction ratio is set to about 2 dB inthe low chirp directly modulated laser.

Here, FIG. 19 schematically shows the light intensity waveform of laserlight output from the DFB laser 100 when the DFB laser 100 is drivenunder such drive conditions.

On the basis of the properties of the DFB laser 100, the wavelength ofoutput light from the DFB laser 100 generates time fluctuation as shownin FIG. 20 in accordance with the light intensity waveform shown in FIG.19 in the case of such high average light output and low extinctionratio drive (this is referred to as a wavelength chirp waveform). Inother words, the laser light (output light) output from the DFB laser100 changes in wavelength approximately in proportion to the lightintensity waveform (the proportion coefficient is a negative value).

For example, when the light intensity of laser light output from the DFBlaser 100 is high, that is, in the case of the ON level, the wavelengthof the laser output light becomes shorter (the frequency becomes higher)and conversely, when the light intensity of laser light output from theDFB laser 100 is low, that is, in the case of the OFF level, thewavelength of the laser output light becomes longer (the frequencybecomes lower). Normally, in a low chirp directly modulated laser, thedrive current amplitude value and the direct current bias current valueare set such that the difference in wavelength of the laser output lightbetween the ON level and the OFF level becomes about 5GHz in terms ofthe frequency.

Additionally, in the case of the normal DML (not the low chirp directlymodulated laser type), not only the change in wavelength in proportionto the light intensity waveform but also a large wavelength fluctuationcomponent in proportion to the time derivative thereof are produced inthe wavelength chirp (fluctuation) waveform in FIG. 20.

In contrast to this, in the case of the low chirp directly modulatedlaser, the wavelength fluctuation component in proportion to the timederivative of such a light intensity waveform is suppressed low by beingdriven under the conditions that the average light output is increasedand the extinction ratio is suppressed low, which results in making asignal more suitable for optical fiber transmission.

By the way, the spectrum of the output light from the DFB laser in thelow chirp directly modulated laser is such one as shown by the solidline A in FIG. 21.

Additionally, in the spectrum shown by the solid line A in FIG. 21, theshort wave component corresponds to the ON level component of the outputlight from the DFB laser and the long wave component corresponds to theOFF level component of the output light from the DFB laser,respectively.

In the low chirp directly modulated laser, signal spectrum processing isperformed for the output light spectrum of such a DFB laser by use of awavelength filter having a wavelength filter function as shown by thesolid line B in FIG. 21. In other words, filtering processing isperformed such that the short wave component corresponding to the ONlevel of the light intensity waveform is transmitted and the long wavecomponent corresponding to the OFF level is cut off.

With such filtering processing, the signal spectrum of the wavelengthfilter transmitted light after passing through the filter and the lightintensity waveform of the wavelength filter transmitted light becomethose shown in FIGS. 22 and 23, respectively.

In other words, as shown in FIG. 22, the signal light of the OFF levelcomponent is cut off relatively more than the signal light of the ONlevel component, therefore, as shown in FIG. 23, the light intensitywaveform of the wavelength filter transmitted light after passingthrough the filter has as large a extinction ratio as about 10 dB incomparison with the light intensity waveform of the output light fromthe DFB laser shown in FIG. 19. Further, as shown in FIG. 22, the widthof the signal spectrum becomes small because the OFF level component hasbeen cut off.

As described above, in the low chirp directly modulated laser, thespectrum width of the signal light is reduced using the wavelengthfilter so that it becomes more unlikely to receive the influence of thegroup velocity dispersion of optical fiber transmission, andlong-distance transmission is made possible, as a result. Further, sincethe extinction ratio of the light intensity waveform to be outputfinally via the wavelength filter is large, it is possible to use thelow chirp directly modulated laser for normal and simple intensitymodulation on off keying (IM-OOK).

SUMMARY OF THE INVENTION

By the way, the low chirp directly modulated laser shown in FIG. 18 iscapable of longer distance transmission than that by a general DML,however, there arises a problem that the configuration of an opticalmodule is complex and the cost is increased.

In comparison with a general DML, the low chirp directly modulated laserrequires an excessive part, that is, the wavelength filter, and tasks ofarranging the DFB laser, the isolator, and the wavelength filter in thisorder, of performing optical axis alignment using, for example, a lens,and of performing optical coupling. Because of this, the assembling cost(the production cost) when manufacturing a module is increased. Further,it also requires a wavelength filter and a lens for performing opticalcoupling, and a space for accommodating them, therefore, there arises aproblem that the total size of a module becomes large.

In order to solve these problems, the low chirp directly modulated laseris demanded to reduce the total production cost and the module size byintegrating these parts to form a single device.

However, in this case, since an isolator capable of being integratedwith a DFB laser does not exist, the above cannot be realized.

On the other hand, an isolator is indispensable for a general DML inorder to prevent cutoff light from a wavelength filter from returning toa DFB laser to adversely affect the operation of the DFB laser.

Because of this, it is desired to realize a low chirp directly modulatedlaser in which a DFB laser and a wavelength filter are integrated toform a single device while preventing cutoff light from the wavelengthfilter from returning to the DFB laser to adversely affect the operationof the DFB laser.

The present invention has been developed in considering such problemsand its object is to provide an optical transmitter capable of longdistance transmission, like a low chirp directly modulated laseraccording to a prior art, and capable of simplifying its configurationand reducing the production cost and the size in comparison with the lowchirp directly modulated laser according to the prior art.

Therefore, an optical transmitter according to the present inventionincludes a directly modulated laser and a wavelength filter provided ona post-stage of the directly modulated laser, wherein the wavelengthfilter has a modulated light input port connected to the directlymodulated laser and inputting modulated light output from the directlymodulated laser, a filter transmitted light output port connected to anoptical coupling system and outputting light having a wavelengthincluded in a filter transmission band among the modulated light asfilter transmitted light, and a filter cutoff light output port providedseparately from the modulated light input port and the filtertransmitted light output port and outputting light having a wavelengthincluded in a filter cutoff band among the modulated light as filtercutoff light, and wherein the peak of the filter transmission band isset on a shorter-wave side from the peak of the spectrum of modulatedlight output from the directly modulated laser.

Therefore, according to the optical transmitter of the presentinvention, long distance transmission is possible, like a low chirpdirectly modulated laser according to the prior art, and there areadvantages that the configuration can be simplified, the production costcan be reduced, and the size can be reduced in comparison with the lowchirp directly modulated laser according to the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram of an optical transmitter according to afirst embodiment of the present invention.

FIG. 2 is a schematic diagram showing a configuration of the opticaltransmitter according to the first embodiment of the present invention.

FIG. 3 is a schematic sectional view showing a configuration of a DFBlaser constituting the optical transmitter according to the firstembodiment of the present invention.

FIG. 4 is a schematic sectional view of a wavelength filter constitutingthe optical transmitter according to the first embodiment of the presentinvention.

FIG. 5 is schematic diagram for explaining a design example of a ringresonator type wavelength filter constituting the optical transmitteraccording to the first embodiment of the present invention.

FIG. 6 is a diagram showing wavelength filter transmission properties ofthe optical transmitter and a modulated spectrum of the DFB laseraccording to the first embodiment of the present invention.

FIG. 7 is a diagram showing an intensity waveform of the DFB laseroutput light of the optical transmitter according to the firstembodiment of the present invention.

FIG. 8 is a diagram showing an intensity waveform of the wavelengthfilter transmitted light of the optical transmitter according to thefirst embodiment of the present invention.

FIG. 9 is a diagram showing an intensity waveform after 80 kmtransmission, using an optical fiber, of the wavelength filtertransmitted light of the optical transmitter according to the firstembodiment of the present invention.

FIG. 10 is a schematic diagram showing a configuration of an opticaltransmitter according to a second embodiment of the present invention.

FIG. 11 is a schematic diagram showing a configuration of an opticaltransmitter according to a third embodiment of the present invention.

FIG. 12 is a schematic diagram showing a configuration of an opticaltransmitter according to a fourth embodiment of the present invention.

FIG. 13 is a schematic diagram showing a configuration of an opticaltransmitter according to a modification example of the fourth embodimentof the present invention.

FIG. 14 is a schematic diagram showing a configuration of an opticaltransmitter according to a fifth embodiment of the present invention.

FIG. 15 is a schematic diagram for explaining a configuration of adiffraction grating of the optical transmitter according to the fifthembodiment of the present invention.

FIG. 16 is a diagram showing wavelength filter transmission propertiesof the optical transmitter and a modulated spectrum of a DFB laseraccording to the fifth embodiment of the present invention.

FIG. 17 is a schematic diagram showing a configuration of an opticaltransmitter according to a sixth embodiment of the present invention.

FIG. 18 is a schematic diagram showing a configuration of a low chirpdirectly modulated laser according to a prior art.

FIG. 19 is a diagram showing an intensity waveform of output light froma DFB laser in the low chirp directly modulated laser according to theprior art.

FIG. 20 is a diagram showing a wavelength chirp waveform of the outputlight from the DFB laser in the low chirp directly modulated laseraccording to the prior art.

FIG. 21 is a diagram showing a spectrum and a wavelength filter functionof the output light from the DFB laser in the low chirp directlymodulated laser according to the prior art.

FIG. 22 is a diagram showing a spectrum of wavelength filter transmittedlight in the low chirp directly modulated laser according to the priorart.

FIG. 23 is a diagram showing an intensity waveform of wavelength filtertransmitted light in the low chirp directly modulated laser according tothe prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The optical transmitter according to the embodiments of the presentinvention will be described below using the drawings.

First Embodiment

First, an optical transmitter according to a first embodiment will bedescribed with reference to FIGS. 1 to 9.

The optical transmitter according to the first embodiment is, forexample, as shown in a conceptual diagram in FIG. 1, a low chirpdirectly modulated laser 1 as an optical transmitter module, including aDFB laser 2 as a single mode oscillation directly modulated laser and awavelength filter 3 provided on a post-stage of the DFB laser 2.

Here, the DFB laser 2 is designed so as to be driven under theconditions of a high average light output (large direct current bias)and a low extinction ratio (small drive current amplitude), like a lowchirp directly modulated laser according to the prior art.

Further, the filter properties of the wavelength filter 3 are set suchthat signal light (modulated light) directly modulated in and outputfrom the DFB laser 2 is inputted thereto via a modulated light inputport 3A, wherein light having a wavelength corresponding to the ON level(light intensity is high) of the modulated light is transmitted andoutput from a filter transmitted light output port 3B, and on the otherhand, light having a wavelength corresponding to the OFF level (lightintensity is low) is cut off and output from a filter cutoff lightoutput port 3C.

In other words, the filter properties of the wavelength filter 3 are setsuch that the light transmittance of a wavelength corresponding to theOFF level is relatively small in comparison with the light transmittanceof a wavelength corresponding to the ON level among the modulated lightfrom the DFB laser 2. Due to this, like the low chirp directly modulatedlaser according to the prior art, the optical transmitter module 1 iscapable of longer distance transmission in comparison with a simple DMLby using, for example, a simple transmission system of IM-OOK.

Particularly, in the present embodiment, the wavelength filter 3 isprovided with the three ports 3A, 3B, and 3C spatially separated andindividually arranged as shown in FIG. 1. In other words, the wavelengthfilter 3 is provided with the modulated light input port 3A, the filtertransmitted light output port 3B, and the filter cutoff light outputport 3C.

The modulated light input port 3A of the ports is one connected to theDFB laser 2 for inputting modulated light (DFB laser output light)output from the DFB laser 2 as filter input light.

Moreover, the filter transmitted light output port 3B is one foroutputting light having a wavelength included in the filter transmissionband among the modulated light as filter transmitted light. The filtertransmitted light output from the filter transmitted light output port3B is coupled to an optical fiber 5 via, for example, an opticalcoupling system such as a coupling lens 4. Thus, the filter transmittedlight output from the filter transmitted light output port 3B is takenout as output light from the optical transmitter module 1. Because ofthis, filter transmitted light is also referred to as module outputlight.

Further, the filter cutoff light output port 3C is one for outputtinglight having a wavelength included in the filter cutoff band among themodulated light as filter cutoff light.

As described above, in the present embodiment, the output light from theDFB laser 2 is directly inputted to the modulated light input port 3A ofthe wavelength filter 3 and the light inputted to the wavelength filter3 is output from the filter transmitted light output port 3B or thefilter cutoff light output port 3C, therefore, even if an isolator isnot provided between the DFB laser 2 and the wavelength filter 3, it isunlikely that the cutoff light cut off by the wavelength filter 3returns to the DFB laser 2 via the input port 3A. Therefore, it is alsounlikely that the DFB laser 2 is adversely affected (the laserproperties are adversely affected) by the returned light from thewavelength filter 3.

In the present embodiment, as shown in FIG. 2, the optical transmittermodule 1 is configured as an integrated device (an integrally formeddevice) in which the DFB laser 2 provided with an active opticalwaveguide and a waveguide type wavelength filter 30 are integrallyformed on the same substrate.

Because of this, tasks such as alignment of optical axis using, forexample, a lens etc. and optical coupling are no longer necessary amongthe DFB laser, the isolator, and the wavelength filter. As a result, theproduction cost and the man-hours can be reduced. Further, by theintegration of devices, the total size of an optical module can bereduced.

Here, as the wavelength filter 30, as shown in FIG. 2, a ring resonatortype wavelength filter is used, which includes a first linear passiveoptical waveguide 30A, a second linear passive optical waveguide 30B, afirst ring-shaped passive optical waveguide 30C, and a secondring-shaped passive optical waveguide 30D.

In the first linear passive optical waveguide 30A of above, one endthereof is connected to the active optical waveguide of the DFB laser 1and the other end extends to the device end surface. In other words, oneend of the first linear optical waveguide 30A constitutes the modulatedlight input port 3A of the wavelength filter 30 and the other endconstitutes the filter cutoff light output port 3C.

The second linear optical waveguide 30B is provided on the opposite sideof the first linear optical waveguide 30A in parallel to the firstlinear optical waveguide 30A, sandwiching the first and secondring-shaped passive optical waveguides 30C and 30D in between. Thesecond linear optical waveguide 30B extends to the device end surfaceand its end constitutes the filter transmitted light output port 3B.Incidentally, the configuration of the first and second ring-shapedpassive optical waveguides 30C and 30D will be described later.

Additionally, to both of the device end surfaces is appliedanti-reflective coating.

A stacking structure of the optical transmitter module (low chirpdirectly modulated laser) 1 as an integrally formed device according tothe present embodiment will be described below with reference to FIGS. 3and 4.

Here, FIG. 3 shows a schematic sectional view of an optical waveguide inthe DFB laser 2. FIG. 4 shows a schematic sectional view of an opticalwaveguide in the ring resonator type wavelength filter 30.

In the present embodiment, as shown in FIGS. 3 and 4, the opticaltransmitter module 1 has a structure in which the DFB laser 2 and thering resonator type wavelength filter 30 are integrated on an n-type InPsubstrate 50.

Here, both the DFB laser 2 and the ring resonator type wavelength filter30 are configured so as to have a ridge type optical waveguidestructure.

First, as shown in FIG. 3, the DFB laser 2 has a structure in which ann-InGaAsP/InP diffraction grating layer 51, an n-InGaAsP guide layer 52,an i (undoped)-InGaAsP related multiple quantum well (MQW) layer 53, ani-InGaAsP guide layer 54, a p-InP cladding layer 55, and a p-InGaAscontact layer 56 are sequentially stacked on the n-type InP substrate50.

Here, the diffraction grating layer 51 has a thickness of, for example,about several tens of nm, and is configured by providing a diffractiongrating in which the n-type InP layer and the n-type InGaAsP layer arearranged alternately in the direction of the optical axis (in thedirection perpendicular to the plane of the paper of FIG. 3). The periodof the diffraction grating is set such that the oscillation wavelengthat the time of continuous drive is approximately 1.55 μm, for example,about 230 nm.

The MQW layer 53 is composed of a six-layer well layer and the welllayer and a barrier layer have thicknesses of about 5 nm and 10 nm,respectively. An emission wavelength of the MQW layer 53 is set to 1.55μm.

The guide layers 52 and 54 are formed by sandwiching the upper and lowersides of the MQW layer 53 and each of their thicknesses is 0.1 μm.

The cladding layer 55 is formed into a mesa shape on the guide layers 52and 54 provided on the upper side of the MQW layer 53 and its width is2.2 μm and its height is 1.3 μm, as shown in FIG. 3.

The contact layer 56 is formed on the top surface of the cladding layer55 formed into a mesa shape.

Additionally, although not shown in FIG. 3, metal electrodes are formedon the entirety of top surface of the contact layer 56 and under surfaceof the n-type InP substrate 50, and the DFB laser 2 is directlymodulated and driven using these electrodes.

Next, the ring resonator type wavelength filter 30 has, as shown in FIG.4, a structure in which an n-InGaAsP guide layer 52A, an n-InGaAsP guidelayer 52B, an i-InGaAsP core layer 57, the p-InP cladding layer 55, andthe p-InGaAs contact layer 56 are sequentially stacked on the n-type InPsubstrate 50.

Incidentally, the composition of the undoped and n-type InGaAsP layers52A, 57, and 54A is made to somewhat differ from that of the DFB laser2. This is for adjusting the equivalent refractive index of the opticalwaveguide constituting the ring resonator type wavelength filter 30 inorder to match it with the equivalent refractive index of the opticalwaveguide constituting the DFB laser 2. For example, the equivalentrefractive index is set to about 3.25. Due to this, an anti-reflectiveeffect can be obtained.

The ring resonator type wavelength filter 30 is not provided with theMQW layer 53 or the upper and lower metal electrodes, which are providedfor the above-mentioned DFB laser 2. Due to this, the ring resonatortype wavelength filter 30 is configured as a passive optical waveguidewith a small transmission loss as a whole.

Additionally, other configurations of the ring resonator type wavelengthfilter 30 are the same as those of the DFB laser 2.

Next, the design of the ring resonator type wavelength filter 30 will bedescribed in detail.

Note that here, the oscillation wavelength of the DFB laser 2 is set toapproximately 1.55 μm and the modulation rate is set to 10 Gb/s.

In the present embodiment, as shown in FIG. 5, the ring resonator typewavelength filter 30 is configured as one in which the two ring-shapedoptical waveguides 30C and 30D are connected in series and the linearoptical waveguides 30A and 30B for input/output are formed on both ofthe sides.

As specific design values, as shown in FIG. 5, each of the equivalentrefractive index of the linear and ring-shaped optical waveguides 30A to30D is set to 3.25, each of the diameter of the two ring-shaped opticalwaveguides 30C and 30D is set to 400 μm, each of the couplingcoefficient between the linear optical waveguides 30A and 30B and thering-shaped optical waveguides 30C and 30D is set to 0.8, and thecoupling coefficient between the ring-shaped optical waveguides 30C and30D is set to 0.32.

When designed as above, the ratio of the power (this is assumed to be100%) of light inputted to the wavelength filter 30 via the first linearoptical waveguide 30A on the input side to the power of light thatcoupled and transferred to the adjoining first ring-shaped opticalwaveguide 30C is about 51% because the coupling coefficient is 0.8.Further, the ratio of the power of light that has been transferred tothe first ring-shaped optical waveguide 30C to the power of light thatcoupled and transferred to the adjoining second ring-shaped opticalwaveguide 30D is about 9.9% because the coupling coefficient is 0.32.Furthermore, the ratio of the power of light that has been transferredto the second ring-shaped optical waveguide 30D to the power of lightthat coupled and transferred to the adjoining second linear opticalwaveguide 30B is about 51% because the coupling coefficient is 0.8.

Here, FIG. 6 shows the calculation result of the transmission properties(cutoff properties) of the ring resonator type wavelength filter 30 asdescribed above.

As is understood from the calculation result in FIG. 6, this ringresonator type wavelength filter 30 has a free spectral range (FSR) ofabout 0.7 nm and a half-value width of about 90 μm.

Further, FIG. 6 also shows a calculation example of the modulatedspectrum of light inputted from the DFB laser 2 to the ring resonatortype wavelength filter 30. Additionally, this is a modulated spectrumwhen the DFB laser 2 is modulated at 10 Gb/s.

As shown in FIG. 6, at a wavelength around which the transmittance fallsby 3 dB, the steepness of the filter cutoff properties to thewavelength, that is, the slope of the filter transmission properties(the change rate of the intensity of the filter transmitted light to thewavelength) results in about 170 dB/nm.

Particularly, in the low chirp directly modulated laser, it is importantto appropriately set the steepness of the cutoff properties of thewavelength filter to the wavelength.

For example, it is preferable to set the steepness of the filter cutoffproperties (the change rate of the intensity of the filter transmittedlight to the wavelength) to, for example, between 100 dB/nm and 300dB/nm at a wavelength around which the transmission light intensityfalls by 3 dB, as described above, for the modulated spectrum of thesignal light output from the DFB laser 2 directly modulated at amodulation rate of 10 Gb/s.

Here, FIG. 7 shows an intensity waveform (eye pattern) of lasermodulated light (signal light) having a DFB laser modulated spectrum(signal spectrum) shown in FIG. 6.

As shown in FIG. 7, laser modulated light from DFB laser has anextinction ratio of about 2 dB. Additionally, it is designed so that thedifference in oscillation wavelength (difference in wavelength of laseroutput light) between the cases of ON level and OFF level is about 5GHz.

By the way, in the present embodiment, as shown in FIG. 6, the center(peak) of the filter transmission band of the wavelength filter 30 isset on a shorter wave side by a predetermined value (here, about 60 pm)to the center (peak) of the modulated spectrum of laser light (signallight) inputted to the wavelength filter 30 from the DFB laser 2.

By performing filtering processing by use of the wavelength filter 30with such properties (that is, after passing through the filter), theintensity waveform (eye pattern) shown in FIG. 7 becomes such one asshown in FIG. 8 and the extinction ratio increases up to about 10 dB.

Then, the signal light (laser modulated light) having the intensitywaveform (eye pattern) as shown in FIG. 7 is transmitted 80 km using ageneral 1.3 μm band zero dispersion fiber (dispersion value=16.7ps/nm·km), and as a result, the intensity waveform (eye pattern) of thesignal light after transmission is as shown in FIG. 9. As describedabove, by being passed through the wavelength filter 30, the extinctionratio increases and the width of the signal spectrum becomes narrow,therefore, it is possible to observe an opening of the excellent eyepattern even after the 80 km transmission.

Consequently, the optical transmitter according to the presentembodiment has advantages that it is capable of long distance opticalfiber transmission, like the low chirp directly modulated laser in theprior art, and that the configuration can be simplified, the productioncost can be reduced, and the size can be reduced in comparison with thelow chirp directly modulated laser in the prior art because the DFBlaser 2 and the ring resonator type wavelength filter 30 are integratedand formed into one device.

Particularly, the present embodiment has an advantage that since thefilter cutoff light is designed to be emitted to the outside (outsidethe device), it is unlikely that the light returns to the DFB laser andadversely affects the operation.

Second Embodiment

Next, an optical transmitter according to a second embodiment of thepresent invention will be explained with reference to FIG. 10.

As shown in FIG. 10, the configuration of the optical transmitter(optical transmitter module; low chirp directly modulated laser)according to the present embodiment resembles the first embodiment(refer to FIG. 2) in that the DFB laser and the ring resonator typewavelength filter are used but differs in that the wavelength filter 30is formed on a planer lightwave circuit (PLC) substrate 60 made of asilica glass material and a DFB laser 2A is integrated in a hybridmanner using the PLC substrate 60 as a platform. Incidentally, in FIG.10, the same components as those in the first embodiment described above(refer to FIG. 2) are assigned with the same symbols.

Additionally, here, as the planer lightwave circuit (PLC) substrate 60,one made of silica glass is used, however, the substrate is not limitedto this, and it is only necessary to use a planer lightwave circuitsubstrate formed of at least one of silica glass, polymer, silicon,dielectric, and semiconductor materials.

Here, the DFB laser 2A is mounted on the PLC substrate 60 such thatthere is a predetermined space between the output end surface of the DFBlaser 2A and the modulated light input port 3A of the PLC substrate 60.Incidentally, the region of surface of the PLC substrate 60 on which theDFB laser 2A is mounted is referred to as a terrace.

Additionally, to the end surfaces of the DFB laser 2A and the PLCsubstrate 60 is applied an anti-reflective coating.

Thus, if the wavelength filter 30 and the DFB laser 2A are formedseparately, it is made possible to optimize them individually.

Here, as the DFB laser 2A, it is only necessary to use one having thesame stacked structure as that in the first embodiment. Also, the DFBlaser 2A maybe configured as that having an buried type opticalwaveguide structure.

Particularly, in the present embodiment, the DFB laser 2A is configuredby providing a spot size enlarged region 2a capable of enlarging a spotsize of laser light (modulated light) at the end surface on the outputside thereof as shown in FIG. 10. Here, the spot size enlarged region 2a is configured by a tapered waveguide. The tapered waveguide may beconfigured such that, for example, the width (or film thickness) of thecladding layer of the optical waveguide constituting the DFB laser 2Abecomes smaller gradually.

In this manner, it is aimed that the output light (modulated light) fromthe DFB laser 2A can be optically coupled to the wavelength filter 30provided on the PLC substrate 60 with high efficiency without providing,for example, a coupling lens.

Further, it is also aimed that laser light (modulated light) is inputtedto the wavelength filter 30 with its spot size enlarged, therefore, itis made possible to optically couple the transmitted light (moduleoutput light) from the wavelength filter 30 directly to the opticalfiber 5 without providing, for example, a coupling lens.

The ring resonator type wavelength filter 30 is configured as one inwhich the two ring-shaped optical waveguides 30C and 30D are connectedin series between the two linear optical waveguides 30A and 30B, likethe first embodiment described above.

However, in the present embodiment, the ring resonator type wavelengthfilter 30 is configured by an optical waveguide made of a silica glassmaterial, therefore, specific design values differ from those in thefirst embodiment.

However, in this case also, it is only necessary to design the diameterand the coupling coefficient of the ring-shaped optical waveguides 30Cand 30D such that the half-value width is about 90 nm, the slope of thecutoff properties at a wavelength around which the transmittance fallsby 3 dB results in 170 dB/nm, and the center of the filter transmissionband is set on a shorter-wave side by about 60 μm from the center of themodulated spectrum of the light output from the DFB laser 2A as theproperties of the wavelength filter 30 obtained finally.

Incidentally, other configurations and operations are the same as thosein the first embodiment described above, therefore, their explanation isomitted here.

Consequently, by using the optical transmitter (low-chirp DML) in thepresent embodiment, long distance optical fiber transmission is madepossible, like the first embodiment described above. Further, since thering resonator type wavelength filter 30 is used and the filter cutofflight is emitted to the outside, it is unlikely that the light returnedto the DFB laser 2A adversely affects the operation of the DFB laser 2A.

Particularly, in the present embodiment, since the DFB laser 2A and thewavelength filter 30 are integrated in a hybrid manner, in comparisonwith those in the first embodiment described above, although theassembling cost of whole devices is somewhat increased, no isolator isrequired and optical coupling using a lens is no longer necessary incomparison with the low-chirp DML in the prior art, therefore, there areadvantages that the total assembling cost can be suppressed low and thetotal size of the optical module can also be reduced.

Third Embodiment

Next, an optical transmitter according to a third embodiment of thepresent invention will be described with reference to FIG. 11.

As shown in FIG. 11, the optical transmitter (optical transmittermodule; low chirp directly modulated laser) according to the presentembodiment differs from the first embodiment described above in that alight absorption region 6 is provided, which is capable of absorbing thefilter cutoff light output from the filter cutoff light output port 3C(or the optical waveguide connected to the filter cutoff light outputport 3C) of the wavelength filter 30. Incidentally, in FIG. 11, the samecomponents as those in the first embodiment (refer to FIG. 2) describedabove are assigned with the same symbols.

Here, in the light absorption region 6, it is only necessary to arrangea material whose absorption end wavelength is positioned on alonger-wavelength side than the longest wavelength of the filter cutofflight so that light having all of the wavelengths included in thespectrum of the filter cutoff light is absorbed.

For example, the light absorption region 6 may be configured as onehaving the same material and the same sectional structure as those ofthe DFB laser 2 (refer to FIG. 3) in the first embodiment describedabove. Then, being different from the DFB laser 2, the potentials of thep-type InGaAs contact layer 56 and the p-type InP cladding layer 55 areset to become smaller than the potential of the n-type InP substrate 50,and the pn junction is kept either at the same potential or at a reversebias voltage.

Incidentally, other configurations and operations are the same as thosein the first embodiment described above, such that the DFB laser 2 andthe ring resonator type wavelength filter 30 are integrated and formedinto one device on the n-type InP substrate 50, therefore, noexplanation is given here.

Consequently, the optical transmitter according to the presentembodiment is capable of long distance optical fiber transmission, likethe first embodiment described above, and has advantages that theconfiguration can be simplified, the production cost can be reduced, andthe size can be reduced in comparison with the low chirp directlymodulated laser in the prior art because the DFB laser 2 and the ringresonator type wavelength filter 30 are integrated and formed into onedevice.

Particularly, in the present embodiment, the possibility that the filtercutoff light cut off by the wavelength filter 30 goes astray afterdischarged (emitted) to the outside (outside the device) is eliminatedand there are advantages that the light is unlikely to adversely affectthe entire operation of the optical transmitter module including the DFBlaser 2 as returned light and that the light is unlikely to be inputtedto the optical fiber arranged ahead of the filter transmitted lightoutput port 3B as noises.

Fourth Embodiment

Next, an optical transmitter according to a fourth embodiment of thepresent invention will be described with reference to FIG. 12.

As shown in FIG. 12, the optical transmitter (optical transmittermodule; low chirp directly modulated laser) according to the presentembodiment differs from the third embodiment described above (refer toFIG. 11) in that the light absorption region 6 is used as a monitorphotodetector (photodetective region) 6A for detecting the filter cutofflight output from the filter cutoff light output port 3C and the amountof wavelength detuning between the center of the oscillation wavelengthof the DFB laser 2 (the spectrum center of the modulated light from theDFB laser 2) and the center of the filter transmission band of thewavelength filter 30 is controlled by monitoring the amount of thefilter cutoff light (photodetective power) and changing the oscillationwavelength of the DFB laser 2. Note that in FIG. 12, the same componentsas those in the third embodiment described above (refer to FIG. 11) areassigned with the same symbols. Due to this, it is made possible torealize a more stable operation as a low-chirp DML.

Because of this, the optical transmitter according to the presentembodiment is provided with, in addition to the configuration of thethird embodiment described above (refer to FIG. 11), as shown in FIG.12, a thin film heater (heater device; temperature control device) 7 inthe DFB laser 2 in order that the temperature of the DFB laser 2 can becontrolled and further provided with a laser drive circuit 8 foroutputting a laser drive electric signal to drive the DFB laser 2, aheater drive circuit 9 for outputting a heater drive electric signal todrive the thin film heater 7, and a control circuit 13 for controllingthe DFB laser 2 and the thin film heater 7 via these drive circuits 8and 9.

Here, the control circuit 13 is connected to the photodetective region6A as a monitor photodetector and designed so as to monitor the amountof filter cutoff light (photodetective power) and perform feedbackcontrol of the drive conditions of the DFB laser 2 (the wavelengthdetuning between the oscillation wavelength of the DFB laser 2 and thewavelength filter 30 is also included) based on this such that the DFBlaser 2 is brought into a preferred state.

Here, a feedback circuit 10 is connected between the DFB laser 2 and thephotodetective region 6A and the control circuit 13 is designed so as toperform drive control (feedback control) of the DFB laser 2 via thelaser drive circuit 8 based on the power of the filter cutoff lightobtained from the photodetective region 6A.

Further, the feedback circuit 10 is connected between the thin filmheater 7 and the photodetective region 6A, and the control circuit 13 isdesigned so as to perform the drive control (feedback control; here,control of the current injected to the thin film heater 7) of the thinfilm heater 7 via the heater drive circuit 9 based on the power of thefilter cutoff light obtained from the photodetective region 6A. Due tothis, the temperature of the DFB laser 2 changes locally and as aresult, the oscillation wavelength of the DFB laser 2 changes and theamount of wavelength detuning between the center of the oscillationwavelength of the DFB laser 2 and the center of the filter transmissionband of the wavelength filter 30 is adjusted.

Incidentally, other configurations and operations are the same as thosein the third embodiment described above, therefore, no explanation isgiven here.

Therefore, the optical transmitter according to the present embodimentis capable of long distance optical fiber transmission, like the firstembodiment described above, and has advantages that the configurationcan be simplified, the production cost can be reduced, and the size canbe reduced in comparison with the low chirp directly modulated laser inthe prior art because the DFB laser 2 and the ring resonator typewavelength filter 30 are integrated and formed into one device.

Particularly, in the present embodiment, the possibility that the filtercutoff light cut off by the wavelength filter 30 goes astray afterdischarged (emitted) to the outside (outside the device) is eliminated,and there are advantages that the light is unlikely to adversely affectthe entire operation of the optical transmitter module including the DFBlaser 2 as returned light and that the light is unlikely to be inputtedto the optical fiber arranged ahead of the filter transmitted lightoutput port 3B as noises.

Further, in the present embodiment, it is possible to control the driveconditions of the DFB laser 2 (the wavelength detuning between theoscillation wavelength of the DFB laser 2 and the cut-off wavelength ofthe filter 30 is also included) such that the DFB laser 2 is broughtinto a preferred state, therefore, it is made possible to realize a morestable operation as a low-chirp DML.

Additionally, in the embodiment described above, the heater 7 isarranged on the DFB laser 2 and wavelength detuning between theoscillation wavelength of the DFB laser 2 and the cut-off wavelength offilter 30 is performed by changing the temperature of the DFB laser 2using the heater 7, however, it is not limited thereto, and for example,a heater may be arranged on the wavelength filter 30 and wavelengthdetuning may be performed by changing the temperature of the wavelengthfilter 30 using the heater [that is, by providing a heater capable ofcontrolling the temperature of the wavelength filter 30 (a temperaturecontrol device)] or heaters may be arranged on both the DFB laser 2 andthe wavelength filter 30 and wavelength detuning may be performed byindividually (independently) changing the temperatures of the DFB laser2 and the wavelength filter 30 using the respective heaters [that is, byproviding heaters capable of controlling both the DFB laser 2 and thewavelength filter 30 (temperature control devices)].

Further, in the embodiment described above, the thin film heater 7 isprovided and it is controlled by the heater drive circuit 9, however, itis not limited thereto.

For example, as shown in FIG. 13, there is provided a Peltier device(cooler device; temperature control device) 11 instead of the thin filmheater 7. Further, there are provided a Peltier device drive circuit 12for outputting an electric signal to drive the Peltier device 11 and thecontrol circuit 13 for controlling the Peltier device 11 via the drivecircuit 12. Then, it may be also possible for the control circuit 13 toperform drive control (feedback control) of the Peltier device 11 viathe Peltier device drive circuit 12 based on the power of the filtercutoff light obtained from the photodetective region 6A. In this case,the whole of a device (integrally formed device) into which the DFBlaser 2 and the wavelength filter 30 are integrally formed is controlledin temperature by the Peltier device 11. In sum, wavelength detuning maybe performed by providing the Peltier device (cooler device; temperaturecontrol device) 11 capable of controlling the temperatures of both theDFB laser 2 and the wavelength filter 30.

In this case, although the effect is not to such an extent as that ofthe above-mentioned embodiments because it is difficult to control thetemperature of the DFB laser 2 independently of the temperature of thewavelength filter 30, the similar effect can be obtained.

Further, it is also possible to configure one in which both the thinfilm heater 7 and the Peltier device 11 are provided by combining theabove-mentioned embodiment (refer to FIG. 12) and the modificationexample (refer to FIG. 13) in which the Peltier device 11 is provided.

Fifth Embodiment

Next an optical transmitter according to a fifth embodiment of thepresent invention will be described with reference to FIG. 14.

The optical transmitter according to the present embodiment has aconfiguration of the wavelength filter different from that in theabove-mentioned first embodiment.

In other words, in the present embodiment, the optical transmitter(optical transmitter module; low chirp directly modulated laser) isconfigured as an integrated device (integrally formed device) in whichthe DFB laser 2 and a diffraction grating mounted Mach-Zehnder (MZ) typewavelength filter 31 as a waveguide type wavelength filter areintegrated on the n-type InP substrate 50 as shown in FIG. 14.Additionally, to both of the end surfaces of the device is applied ananti-reflective coating. Incidentally, in FIG. 14, the same componentsas those in the above-mentioned first embodiment (refer to FIG. 2) areassigned with the same symbols.

Here, the diffraction grating mounted MZ type wavelength filter 31 isconfigured by including a Mach-Zehnder interferometer 36 having twolinear optical waveguides (arms) 32 and 33 and diffraction gratings(gratings; diffraction grating type wavelength filters) 37 and 38 formedon the respective two linear optical waveguides 32 and 33 in theMach-Zehnder interferometer 36. Additionally, the diffraction gratings37 and 38 provided on the respective two optical waveguides 32 and 33 inthe Mach-Zehnder interferometer 36 have the same structure.

Further, the Mach-Zehnder interferometer 36 is provided with two multimode interference (MMI) photocouplers 34 and 35.

Connected to the MMI photocoupler 34 on a pre-stage constituting theMach-Zehnder interferometer 36 are two optical waveguides on its inputside. The end of the optical waveguide on one side constitutes themodulated light input port 3A and the end of the other optical waveguideconstitutes the filter cutoff light output port 3C. Then, to themodulated light input port 3A, the DFB laser 2 is connected via amodulated light input optical waveguide 40. Connected to the filtercutoff light output port 3C is a filter cutoff light output opticalwaveguide 41 extending to the device end surface.

Further, to the MMI photocoupler 35 on a post-stage constituting theMach-Zehnder interferometer 36, a filter transmitted light outputoptical waveguide 42 extending to the device end surface is connected onits output side, and the end of the filter transmitted light outputoptical waveguide 42 constitutes the filter transmitted light outputport 3B.

Thus, in the present embodiment, the diffraction grating mounted MZ typewavelength filter 31 results in a configuration in which the threeports: the modulated light input port 3A, the filter transmitted lightoutput port 3B, and the filter cutoff light output port 3C are spatiallyindividually arranged.

Since configured as above, the modulated light output from the DFB laser2 is inputted to the modulated light input port 3A of the wavelengthfilter 31 via the modulated light input optical waveguide 40, branchedby the MMI photocoupler 34 on the pre-stage of the wavelength filter 31,and guided to the respective diffraction gratings 37 and 38 through thelinear optical waveguides 32 and 33. Then, light having a wavelengthincluded in the filter transmission band of the diffraction gratings 37and 38 (filter transmitted light) passes through the respectivediffraction gratings 37 and 38, multiplexed to each other by the MMIphotocoupler 35 on the post-stage, and output as filter transmittedlight (module output light) from the filter transmitted light outputport 3B via the filter transmitted light output optical waveguide 42connected to the MMI photocoupler 35 on the post-stage.

Then, the filter transmitted light output from the filter transmittedlight output port 3B is designed to be coupled to the optical fiber 5via the coupling lens 4.

On the other hand, light having a wavelength included in the filtercutoff band of the diffraction gratings 37 and 38 (filter cutoff light)is cut off by the respective diffraction gratings 37 and 38. In otherwords, the light cut off and reflected by the diffraction gratings 37and 38 returns to the MMI photocoupler 34 on the pre-stage to bemultiplexed and output as filter cutoff light to the outside from thefilter cutoff light output port (other than the modulated light inputport and the filter transmitted light output port) via the filter cutofflight output optical waveguide 41.

In the present embodiment, both the DFB laser 2 and the diffractiongrating mounted MZ type wavelength filter 31 are configured as an buriedtype optical waveguide.

First, the core of the optical waveguide constituting the DFB laser 2 isformed by an InGaAsP related multiple quantum well (MQW) layer andconfigured as one having an emission wavelength of, for example, 1.55μm.

On the other hand, the core of the optical waveguide constituting thediffraction grating mounted MZ type wavelength filter 31 is formed of amaterial having an emission wavelength of shorter waves in comparisonwith the optical waveguide constituting the DFB laser 2. For example, itmay be formed of a bulk InGaAsP layer etc.

Here, FIG. 15 schematically shows the structure of the diffractiongrating 37 (38) formed in the Mach-Zehnder interferometer 36.Additionally, FIG. 15 shows only the diffraction grating 37 provided onthe optical waveguide 32 on one side for convenience of explanation.

As shown in FIG. 15, the diffraction grating 37 is formed by alternatelyforming a high refractive index part 45 and a low refractive index part46 on a part of the optical waveguide 32. The total length of the parton which the diffraction grating 37 of the optical waveguide 32 isprovided is set to 800 μm. All of the diffraction gratings 37 areuniformly manufactured except λ/4 phase shifts 47 and 48 provided at twopositions in FIG. 15. The value of the coupling coefficient K of thediffraction grating 37 is set to 89/cm and the period thereof is set to238 nm. The average equivalent refractive index of the optical waveguide32 at the part on which the diffraction grating 37 is provided is 3.25.

The transmission spectrum (transmission properties; cutoff properties)of the diffraction grating mounted Mach-Zehnder type wavelength filter31 configured using such a diffraction grating 37 is as shown in FIG.16. Additionally, FIG. 16 also shows a calculation example of themodulated spectrum of light to be inputted from the DFB laser 2 to thediffraction grating mounted Mach-Zehnder type wavelength filter 31.

The half-value width of the peak of the transmission properties of thewavelength filter 31 obtained from the wavelength filter transmissionproperties shown in FIG. 16 is about 90 pm like that in theabove-mentioned first embodiment (refer to FIG. 6).

As shown in FIG. 16, at a wavelength around which the transmittancefalls by 3 dB, the steepness of the filter cutoff properties to thewavelength (the change rate of the intensity of the filter transmittedlight to the wavelength) results in also about 170 db/nm, like that inthe above-mentioned first embodiment.

Further, as shown in FIG. 16, in the present embodiment also, like theabove-mentioned first embodiment, the center (peak) of the filtertransmission band of the wavelength filter 31 is set on a shorter-waveside by a predetermined value (here, about 60 pm) to the center (peak)of the modulated spectrum of the laser light (signal light) to beinputted from the DFB laser 2 to the wavelength filter 31.

Thus, since the filter properties are set like the above-mentioned firstembodiment, in the present embodiment also, the same eye pattern (referto FIG. 8) of the filter transmitted light as that in theabove-mentioned first embodiment and the eye pattern (refer to FIG. 9)after 80 km transmission using the optical fiber can be obtained.

Incidentally, since other configurations etc. are the same as those inthe above-mentioned first embodiment, no explanation is given here.

Therefore, according to the optical transmitter of the presentembodiment, the same functions and effects as those in theabove-mentioned first embodiment can be obtained. In other words, longdistance optical fiber transmission is made possible while realizing anintegrally formed simple configuration by integrating the DFB laser 2and the wavelength filter 31. Further, it is unlikely that the filtercutoff light acts as returned light and adversely affects the operationof the DFB laser 2.

Sixth Embodiment

Next, an optical transmitter according to a sixth embodiment of thepresent invention will be described with reference to FIG. 17.

The optical transmitter according to the present embodiment resemblesthe above-mentioned fifth embodiment in including the Mach-Zehnderinterferometer type wavelength filter but differs in that a ringresonator is provided in the vicinity of the linear optical waveguideson one side instead of the diffraction grating.

In other words, in the present embodiment, the optical transmitter(optical transmitter module; low chirp directly modulated laser) isconfigured as an integrated device (integrally formed device) in whichthe DFB laser 2 and the ring resonator mounted Mach-Zehnder (MZ) typewavelength filter 43 as a waveguide type wavelength filter areintegrated on the n-type InP substrate 50 as shown in FIG. 17.Incidentally, in FIG. 17, the same components as those in theabove-mentioned fifth embodiment (refer to FIG. 14) are assigned withthe same symbols.

Here, the ring resonator mounted MZ type wavelength filter 43 isconfigured by including the Mach-Zehnder interferometer 36 provided withthe two linear optical waveguides (arms) 32 and 33 and the ringresonator (ring resonator type wavelength filter) 49 provided in thevicinity of the linear optical waveguide 33 on one side of theMach-Zehnder interferometer 36.

Further, the Mach-Zehnder interferometer 36 is provided with two multimode interference (MMI) photocouplers 34 and 35.

To the MMI photocoupler 34 on the pre-stage constituting theMach-Zehnder interferometer 36, the optical waveguide is connected onits input side and the end of the optical waveguide constitutes themodulated light input port 3A. To the modulated light input port 3A, theDFB laser 2 is connected via the modulated light input optical waveguide40.

Further, to the MMI photocoupler 35 on the post-stage constituting theMach-Zehnder interferometer 36, the two optical waveguides: the filtercutoff light output optical waveguide 41 extending to the device endsurface and the filter transmitted light output optical waveguide 42extending to the device end surface are connected on its output side.Then, the end of the filter cutoff light output optical waveguide 41constitutes the filter cutoff light output port 3C and the end of thefilter transmitted light output optical waveguide 42 constitutes thefilter transmitted light output port 3B.

Thus, in the present embodiment, the ring resonator mounted MZ typewavelength filter 43 results in a configuration in which the threeports: the modulated light input port 3A, the filter transmitted lightoutput port 3B, and the filter cutoff light output port 3C are spatiallyindividually arranged.

Due to such a configuration, the modulated light output from the DFBlaser 2 is inputted to the modulated light input port 3A of thewavelength filter 32 via the modulated light input optical waveguide 40and branched into two by the MMI photocoupler 34 on the pre-stage of thewavelength filter 31.

One of the branched light is inputted to the ring resonator (ringresonator type filter) 49 via the linear optical waveguide 33. In thiscase, light having a wavelength included in the filter transmission bandof the ring resonator 49 (filter transmitted light; light having awavelength in the vicinity of the resonant wavelength of the ringresonator) passes through the ring resonator. On the other hand, theother branched light passes through the linear optical waveguide 32 asit is. Then, they are multiplexed by the MMI photocoupler 35 on thepost-stage and output as filter transmitted light (module output light)from the filter transmitted light output port 3B.

Thus, the ring resonator mounted Mach Zehnder interferometer arranged onthe post-stage of the DFB laser 2 functions as a wavelength filter.Particularly, by optimizing the structure of the ring resonator, it ispossible for this wavelength filter to realize the steep transmissionproperties (refer to FIG. 16) equivalent to those in the above-mentionedfifth embodiment.

Then, the filter transmitted light output from the filter transmittedlight output port 3B is coupled to the optical fiber 5 via the couplinglens 4.

On the other hand, as described above, when the light having propagatedthrough the respective linear optical waveguides 32 and 33 ismultiplexed by the MMI photocoupler 35 on the post-stage, light having awavelength included in the filter cutoff band of the ring resonator(filter cutoff light; light having a wavelength other than thewavelength in the vicinity of the resonant wavelength of the ringresonator) is output as filter cutoff light from the filter cutoff lightoutput port (other than the modulated light input port 3A and the filtertransmitted light output port 3B) to the outside.

Thus, the cutoff light by the wavelength filter 43 is output from theport (the filter cutoff light output port 3C) other than the modulatedlight input port 3A, therefore, it is unlikely that it becomes returnedlight to the DFB laser 2.

Incidentally, since other configurations etc. are the same as those inthe above-mentioned fifth embodiment, no explanation is given here.

Therefore, according to the present embodiment, the same functions andeffects as those in the above-mentioned fifth embodiment can beobtained. In other words, long distance optical fiber transmission ismade possible while realizing an integrally formed simple configurationby integrating the DFB laser 2 and the wavelength filter 43. Further, itis unlikely that the filter cutoff light becomes returned light andadversely affects the operation of the DFB laser 2.

Others

The filter structure, the details of the settings of the transmissionproperties, etc. are not limited to the configuration of each embodimentdescribed above. For example, the order (number of stages), thediameter, and the coupling coefficient of the ring resonator, the numberof phase shifts, the position, the total length, and the couplingcoefficient of the diffraction grating, the details of the filterproperties of the half-value width, the steepness of the cutoffproperties, the free spectral range (FSR), etc., realized by thesefilter structures can be appropriately optimized by the properties ofthe operation wavelengths of the directly modulated laser, thetransmission distance of the optical transmitter module, the modulationrate, the required extinction ratio, etc.

Further, each embodiment described above may be arbitrarily combined.For example, the characteristic structures in the third and fourthembodiments, in which the light absorption region and the photodetectiveregion are provided, may be combined with those in the fifth and sixthembodiments.

Furthermore, the present invention is not limited to each of theembodiments described above, and there can be various modificationswithin the scope of the present invention without departing from theconcept thereof.

1. An optical transmitter comprising: a directly modulated laser; and awavelength filter provided on a post-stage of said directly modulatedlaser, wherein, said wavelength filter has: a modulated light input portconnected to said directly modulated laser and inputting modulated lightoutput from said directly modulated laser; a filter transmitted lightoutput port connected to an optical coupling system and outputting lighthaving a wavelength included in a filter transmission band among saidmodulated light as filter transmitted light; and a filter cutoff lightoutput port provided separately from said modulated light input port andsaid filter transmitted light output port and outputting light having awavelength included in a filter cutoff band among said modulated lightas filter cutoff light, and wherein the peak of said filter transmissionband is set on a shorter-wave side from the peak of the spectrum ofmodulated light output from said directly modulated laser.
 2. Theoptical transmitter according to claim 1, wherein said directlymodulated laser and said wavelength filter are integrally formed on thesame substrate.
 3. The optical transmitter according to claim 1, whereinsaid directly modulated laser and said wavelength filter are integratedin a hybrid manner on a planer lightwave circuit substrate formed of oneof silica glass, polymer, silicon, dielectric, and semiconductormaterials.
 4. The optical transmitter according to claim 1, comprising alight absorption region for absorbing filter cutoff light output fromsaid filter cutoff light output port.
 5. The optical transmitteraccording to claim 1, comprising a temperature control device capable ofcontrolling the temperature of said directly modulated laser and/or saidwavelength filter.
 6. The optical transmitter according to claim 1,further comprising: a monitor photodetector for detecting filter cutofflight output from said filter cutoff light output port; and a controlcircuit for controlling said directly modulated laser based on the powerof filter cutoff light detected by said monitor photodetector.
 7. Theoptical transmitter according to claim 5, further comprising: a monitorphotodetector for detecting filter cutoff light output from said filtercutoff light output port; and a control circuit for controlling saidtemperature control device based on the power of filter cutoff lightdetected by said monitor photodetector.
 8. The optical transmitteraccording to claim 1, wherein said wavelength filter is a ring resonatortype wavelength filter formed of a ring resonator.
 9. The opticaltransmitter according to claim 1, wherein said wavelength filter is aMach-Zehnder type wavelength filter formed of a Mach-Zehnderinterferometer.
 10. The optical transmitter according to claim 9,wherein said Mach-Zehnder type wavelength filter is a diffractiongrating mounted Mach-Zehnder type wavelength filter that mounts adiffraction grating on said Mach-Zehnder interferometer.
 11. The opticaltransmitter according to claim 9, wherein said Mach-Zehnder typewavelength filter is a ring resonator mounted Mach-Zehnder typewavelength filter that mounts a ring resonator on said Mach-Zehnderinterferometer.
 12. The optical transmitter according to claim 1,wherein said wavelength filter is composed such that the change rate ofthe intensity of said filter transmitted light with respect to thewavelength is in a range of 100 dB/nm to 300 dB/nm at a wavelength atwhich the intensity of said filter transmitted light falls by 3 dB.